Monoliths manufactured using corrugated thin metal foils are being used in a number of applications, e.g., as selective catalytic reduction (SCR) units in power plant exhaust lines, or in catalytic converters for stationary or automobile engine exhaust lines. Of particular interest are those monoliths which are of a flow-through design. The term "flow-through" describes a structure which has fluid flow passages, e.g., exhaust gas flow passages, which begin at one end of the unit and which end at the opposite end of the unit in a continuous manner. The length of the passage ways is not usually longer than the length of the monolith. If the corrugations are oblique, albeit straight through, the passageway will be slightly longer than the length of the monolith.
These structures are most commonly applied in the catalytic removal of pollutants in automotive exhaust, diesel exhaust, coal exhaust, glass plant exhaust, ozone removal, etc. Each application requires a different catalytic approach, consideration of the operating conditions of the catalyst, and the flow resistance or pressure drop through the unit.
Metal based monoliths can be made by corrugating thin metal strips of foil and stacking it either upon itself (with nothing between successive layers) or by separating the layers with flat foil. (See FIG. 1). The latter technique is often used with foil corrugated with a uniform, nestable, wave pattern (where the term "nestable" implies that two contiguous layers of this foil, if oriented in the usual way will nest completely together leaving no space between the contacting surfaces. This sort of pattern is useless for building up a monolithic honeycomb structure unless the successive layers are held apart by means of spacers, or typically, flat foil as shown in FIG. 1.
Another type of monolith is one built up using a corrugation which runs at an acute angle, usually 3.degree.-10.degree., relative to a line orthogonal to the length of the foil strip. The corrugation may either continue across the width of the foil, or may be discontinuous or interrupted in a herringbone fashion across the width of the foil. This type of corrugated foil will not nest if folded back upon itself (See FIG. 2).
There are many diffrent combinations of these two basic structures which will not nest on layering. Some of these are:
(1) Herringbone or angled patterns folded back upon themselves will not nest.
(2) Alternating flat foil and straight corrugated foil will yield a nonnesting structure.
(3) Alternating flat and any type of corrugated foil will yield a nonnesting structure.
(4) Stacking alternating straight and then herringbone corrugated foils will yield a nonnesting structure.
(5) Stacking straight corrugations of differing wavelengths and/or amplitudes will yield partially nesting structures.
Structures (1) and (2) above are the most common of the flow-through monoliths, and each has its own particular advantages. The herringbone foil folded back upon itself is an excellent mass transfer structure because of the turbulence induced by the periodic flow interruptions. This is significant in that smaller catalyst surface is required for a given application as compared to a straight through cell catalyst surface of equal channel size. It is also easier to manufacture the herringbone type of foil than it is to make the alternating flat foil with a straight cell foil, because it requires but one type of foil to produce a monolith.
The alternating flat on straight corrugated monoliths, have lower pressure drop than the herringbone monoliths because they have straight channels which do not periodically interrupt the flow. These straight channels do not clog as easily in sooty environments for the same reason, i.e., particles tend to slip through rather than contact the walls of the monolith. They are more difficult to produce than the herringbone monoliths especially if the shape of the monolith is noncircular. The typical manufacturing technique is winding the foil in a spiral.
From the previous discussion, a desirable structure is one which requires a single type of foil, and which monolith can be produced by simply folding the corrugated foil back and forth upon itself in an accordion fold manner to provide layers of any desired cross-section, including circular, oval, or rectangular, or sections of any of the foregoing. It is, therefore, a primary objective of this invention to provide a structure and a method of making the same, characterized by a straight celled monolith having corrugations which do not nest when the corrugated foil is folded back upon itself, or accordion folded.
For convenience, the following terms as used herein will have the meanings ascribed to them:
Straight Cells. Straight cells are those formed from corrugations which run at a constant angle (e.g. 80.degree.-90.degree.) to the longitudinal marginal edges of the foil. PA0 Wavelength. Wavelength is the length of the repeating unit in a corrugation pattern. PA0 Amplitude. The amplitude of a corrugation is the vertical height of such corrugation from its base to its peak. PA0 Pattern. A pattern is the longitudinal length of corrugations of a given amplitude and wavelength. PA0 Repeating Unit. A repeating unit is a string of patterns, or a single pattern, which repeats periodically in phase, amplitude and wavelength. PA0 Wave Type. The wave type is the geometric configuration of the wave and may be sinusoidal, square, triangular, trapezoidal, semicircular, etc. PA0 Chord. A chord is the longitudinal span of corrugated foil in a layer, whether or not accordion folded, or in individual layers extending from one side to the other of a given geometric shape of the monolith. PA0 Contacting Point. A contacting point in a multi-wavelength design is the point where contiguous layers have less than a predetermined nesting fraction. PA0 Nesting Fraction. The maximum amount of nesting which can occur at a contacting point. PA0 Zero Point. The zero point is on the line drawn through the corrugation pattern at a value equal to one half the amplitude.